Hydraulic cement compositions have, for a long time now, frequently been employed in the construction of buildings, in civil engineering work such as structures (bridges, dams, roads, motorways, tunnels, reservoirs, etc.), but also in geological excavations, such as, for example, oil wells, water wells or any other type of well, for cementing casings, and liners, for the production of cement plugs and for injection under pressure. In in many cases, it can happen that the environment to be cemented is at a higher temperature than the usual application temperature, with the result that the time required for normal setting of the cement is greatly reduced, thus exhibiting an increase in the rate of setting of the cement, which is awkward for the specialist. Under these conditions, it becomes difficult to use a hydraulic cement the physical state of which undergoes an unfavourable change with the thermal state of the environment to be cemented. In fact, this problem of controlling the setting of hydraulic cements, when they are employed in media at a temperature which is higher than normal (accelerating the setting rate), has not yet been fully solved.
In the particular case of oil prospecting, this problem is even more acute because in drilling for oil the cementing operation is carried out by injecting the hydraulic cement composition, according to techniques which are well known to the expert, in order to place this composition in contact with the walls to be cemented. Now, in boreholes the temperature increases with the depth and, consequently, conventional cements cannot be employed on their own because when they are subjected to this temperature rise, their setting is accelerated too much for the placing of the cement to be carried out correctly. This is why their setting must be delayed so that the upward transfer may take place without an increase in viscosity, that the annulus may be cemented over the entire depth of the borehole and that these walls may be leakproof after the cementing.
Another problem which is added to this problem of acceleration of cement setting due to a rise in the temperature of the environment, is that of anticipating and bringing under control the behaviour of the hydraulic cement composition as it passes through diverse geological formations. The initial rheological properties of the said composition, together with other physical properties, may be affected by the mineral materials forming the layers through which it passes, such as saline layers, or by the inflow of water, particularly brines.
Furthermore, the development of offshore oil prospecting renders this problem more difficult because the cement compositions employed must be capable of being prepared with the use of brine (seawater).
For the expert, known cement compositions consist firstly of a hydraulic cement, such as, for example, a Portland cement, an alumina cement, and then freshwater or seawater or saturated brine and at least one adjuvant enabling some properties of the cement compositions employed to be controlled, if possible, without these properties being modified by the difficult temperature and pressure conditions prevailing at the bottom of the well.
Among these properties which the specialist considers to be essential, the hydraulic cement composition must retain for several hours its pumpability, offer good rheological properties, prevent the loss of liquid by filtration through porous formations and, lastly, have good compressive strength.
This is why many chemical adjuvants have so far been suggested and described in specialist literature, to try to provide a solution to the abovementioned problems or phenomena.
A first type of adjuvant, which is a setting retarder for a hydraulic cement composition (Portland cement) is recommended in U.S. Pat. No. 4,054,460. This adjuvant consists mostly of CaF.sub.2 and of smaller amounts of Ca.sub.3 (PO.sub.4).sub.2, CaSo.sub.4, SiO.sub.2, Fe.sub.2 O.sub.3 and Al.sub.2 O.sub.3. While an adjuvant such as this appears to fulfil the function of a setting retarder in environments subject to ambient temperature of the order of 20.degree. C. and 35.degree. C. it is found to be ineffective when the temperature "in situ" attains high levels, such as for example 150.degree. C., at the bottom of a borehole.
Another type of a setting-retarder adjuvant for a hydraulic cement composition at a high temperature (not exceeding 400.degree. F., that is to say not exceeding 204.degree. C.) is described in Canadian Pat. No. 970,398. This adjuvant is found to be a synergistic combination of lignosulphonic acids (or of a water-soluble salt) and boric acid (or of a water-soluble salt). However, while such a combination provides this function of retarding cement setting at a temperature as high as 204.degree. C. (400.degree. F.) because boric acid reinforces the widely known setting-retardant effect of lignosulphonic acid, it has major disadvantages, one of which relates to its utilization exclusively in a freshwater environment, another is that the setting-retardant effect is produced by two chemical compounds which must be strictly adjusted relative to each other for each cement composition, and that, finally, one of the greatest of these is their contaminating nature with regard to the environment in the case of chromium lignosulfonates or iron chromium lignosulfonates.
In a manner which is closely related to the adjuvant described in the abovementioned Canadian patent, U.S. Pat. No. 3,856,541 proposes as a setting retarder for a hydraulic cement composition subjected to a relatively high temperature (193.degree. C.) another synergistic combination comprising a boron compound in a salt form, for example, and a hydroxycarboxylic acid such as tartaric acid, citric acid, gluconic acid or glucoheptonic acid, this synergistic combination having substantially the same disadvantages as those already mentioned in respect of the Canadian patent, that is to say that such a setting-retardant (and nonplasticizing) combination can be employed only in a freshwater environment and that this effect is produced by a mixture of two compounds, the boron compound reinforcing the hydroxycarboxylic acid, which would substantially lose its retardant effect if it was on its own when the said cement composition was employed in environments at a high temperature.
A latest type of setting-retarder adjuvant for a hydraulic cement composition is described in British Pat. No. 2,031,397. This adjuvant consists of polymers of a molecular weight of between 50,000 and 500,000, comprising anionic structures (acrylic and methacrylic acids) and nonionic structures (acrylamide, methacrylamide). However, the behaviour of this adjuvant as a setting retarder for cement does not appear to have been verified beyond a temperature threshold of 60.degree. C., a temperature level which is too low in the case of the cementation of oil boreholes. And, furthermore, these acrylic acid derivatives are generally better known as thickening agents, for example in drilling fluids; a product of the acrylamide/sodium acrylate or substituent type is described as a thickening agent in U.S. Pat. No. 4,059,552.
Finally, as U.S. Pat. No. 3,952,805 teaches, it is recommended to use a copolymer of maleic anhydride and styrenesulphonic acid as a plasticizing agent for a hydraulic cement composition, even in a saline aqueous environment. However, such a composition does not appear to provide any retardant effect whatever in respect of these cement compositions.
Thus, the prior art offers the specialist solutions which cannot fully satisfy him because the recommended chemical adjuvants are frequently disappointing in their use, because their effect is weakened either by the presence of interfering inorganic compounds such as, for example, sodium chloride, etc., or owing to the temperature change phenomenon such as that which occurs at the bottom of oil boreholes, or else because these adjuvants become inefficient or are inhibited when they are introduced into a hydraulic cement suspension in which the aqueous phase is saline, or, finally, because these adjuvants can be of an environmentally polluting nature.